An ion trap for generating a radially symmetric three dimensional trapping field is shown in FIG. 1. The trap includes three parts: a ring electrode 11 and two end caps 12 and 13. The interior facing surfaces of the electrodes have appropriate hyperbolic shape. The end electrode 12 is associated with an electron gun which projects ionizing electrons through the opening 14. The electron gun includes a filament assembly 15, a lens 16 and a gate 17. Ions are analyzed by exciting the ions such that they are ejected through the opening 18 formed in any cap 15. The ions are detected by the electron multiplier 19. See U.S. Pat. Nos. 4,540,884 and Reissue 3400 for operation of an ion trap as a mass spectrometer. A three dimensional ion trapping field is generated by applying rf and dc or rf only voltages to the trap electrodes. The Mathieu equations of motion of ions in a three dimensional trapping field define the voltages required to capture ions over a selected mass range. A discussion of the principles of operation of the quadrupole ion trap can be found in the book "Quadrupole Mass Spectrometry and its Applications" edited by Peter Dawson, pages 49-52 and 134-188. Radio frequency quadrupole ion traps are also discussed in chapter 4, pages 39-49 of the book Dynamic Mass Spectrometry, Vol. 4, edited by D. Price and J. F. J. Todd and the book by R. E. March and R. J. Hughes (Quadrupole Storage Mass Spectrometry), Wiley-Interscience, New York, 1989.
U.S. Pat. No. 4,755,670 describes a method of analyzing a wide mass range of ions captured in a quadrupole ion trap. The patent describes a method of forming and trapping a wide mass range of ions in a quadrupole field, exciting the ions by applying an electrical pulse to the ions such that it imparts into the ions coherent motion. The ion motion induces image currents in the end caps. The magnitude and frequency of the image current is proportional to the frequency and magnitude of the ions oscillating trajectory. The ion image current is then frequency analyzed and a frequency spectrum which corresponds to the mass spectrum is obtained.
Parks et al. [J. H. Parks, S. Pollack, and W. Hill "Cluster experiments in radio frequency Paul traps: Collisional relaxation and dissociation", J. Chem. Phys. 101 (8), 1994] demonstrated non-destructive detection of trapped C.sub.60 +ions with a detection sensitivity of&lt;100 ions. The detection circuit connected to the end caps was designed for narrow band detection. Detection was accomplished at the resonance ion frequency using a resonant LC circuit with a sufficiently narrow bandwidth (.omega..sub.z /.omega.=200) to achieve low noise in the presence of large background signals at the rf drive frequency.
Goefinger et al. [D. E. Goefinger, R. I. Crutcher, and S. A. McLuckey "Ion remeasurement in the Radio Frequency Quadrupole Ion Trap", Anal. Chem. 67, 1995, 4164-4169] demonstrated narrow band non-destructive detection of ions in a quadrupole ion trap and ion remeasurement. The experiment was done by simultaneous excitation and image current detection using the ion trap electrodes. These authors used a transformer-coupled bridge circuit in order to circumvent a large contribution to the signal from the rf drive frequency in the detector signal.
The use of the ion trap electrodes to detect the image currents introduces a large capacitance which decreases the noise ratio.